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Abstract

Dopamine (DA) D2 receptor expression parallels DA levels in the brain and these autoreceptors have been shown to be modulated by long-term ethanol exposure. We have previously demonstrated that ventral tegmental area (VTA) GABA neurons also express D2 autoreceptors (D2R), and that DA and D2R agonists markedly enhance the excitability of VTA GABA neurons, opposite to their well-known auto-receptor inhibition of DA neurons. Most importantly, D2R antagonists block ethanol inhibition of VTA GABA neurons and D2R expression in VTA GABA neurons down-regulates with chronic ethanol, as others have shown for whole VTA D2R expression. The aim of this study was to evaluate short-term D2R adaptation in specific brain reward regions i.e., ventral tegmental area (VTA), nucleus accumbens (NAc), temporal lobe cortex, and also in peripheral white blood cells (WBCs) as a potential biomarker for brain DA. To accomplish these studies, we used quantitative RT-PCR to analyze rapid (within 2 hrs) changes in D2R expression from both brain and blood samples of rats from one of four in vivo treatment groups: saline, ethanol (2.5 g/kg, IP), eticlopride (1 mg/kg, IV), or quinpirole (0.1 mg/kg, IV). To verify the qRT-PCR effect we observed from tissue punches of the selected brain regions, we used immunofluorescence to quantify changes in D2R expression between the four treatment groups. To determine whether D2R adaptation in the blood was dependent on communication with the brain, we extracted blood samples and performed the same type of in vivo experiments in vitro. We found that D2R expression was increased in the VTA with ethanol, eticlopride and quinpirole, increased in the NAc with ethanol but decreased with eticlopride and quinpirole, and decreased with ethanol and quinpirole in the temporal lobe cortex. In the in vivo blood experiments, D2R expression decreased in WBCs in all three drug treatment groups. In vitro blood experiments showed increased expression with ethanol treatment and decreased with eticlopride. When compared to saline treated animals, the immunofluorescence in the VTA suggests that D2R expression increased in ethanol and eticlopride, but decreased in quinpirole treated animals. At this point, it is clear that D2R expression shows rapid adaptation when exposed to acute doses of ethanol and D2 targeting drugs in both the brain and blood. More evidence is needed through in vitro studies to determine whether a specific neuro-immune interaction is directing the changes seen in the blood and whether or not chronically exposed animals show significantly decreased D2R expression in the blood.